PSI - Issue 39

Costanzo Bellini et al. / Procedia Structural Integrity 39 (2022) 574–581 Author name / Structural Integrity Procedia 00 (2019) 000–000

579

6

The addition of titanium in the pure zinc bath, up to a value of 0.5%, determines not only a greater reactivity of the zinc bath on the surface of the steel, with a consequent increase in the thickness of the coatings, but also a different formation of the intermetallic phases. As can be seen in Fig. 6, the coatings obtained from this bath present not only a highly developed iron- rich δ phase (high reactivity of the bath), but also a matrix characterised by the presence of a compact but ductile phase, similar to the η phase, with the presence of a lamellar phase.

Lamellar phases Matrix

Band of τ phase

τ phase δ phase

Band of τ phase

Fig. 6. Microstructure of coatings obtained from Zn-Ti 0.5% .

In addition, within this matrix it is possible to observe the presence of a compact phase, rich in iron and titanium, called the τ phase. This phase develops from the δ phase due to titan ium enrichment and detaches from it, migrating into the compact matrix to form different layers, especially at high immersion times. This phase, especially in the outermost parts, can be dispersed in the bath if it reaches the surface or it releases titanium to the compact matrix, forming the lamellar phase. The main characteristic of t he τ phase is its high hardness. In all the investigated coatings the radial crack of δ phases is the main damage of coatings in the tensile side of specimens, and for this reason the number of radial cracks for deformed arc length has been chosen as damage parameter. In figure 7, the damages of δ phases of coatings obtained for 180 s of dipping time for each investigated bath is shown.It is possible to understand as the bending damage sharply increases in the case of coatings obtained from pure Zn baths. The presence of Sn smooths the increment of damage at a higher bending angle, improving the ductility of the coating. The minimum damage is obtained in the coatings with Cu addition. This is due to the increase of toughness as an effect of Cu addition. The coatings containing Ti are characterized by an increment of damage at a high value of deformation similar to the damage observed in coatings containing Sn. 4. Conclusion In this work, four different bath chemical compositions have been used to obtain galvanized coatings. Specimens have been tested by traditional bending tests and sections of coatings have been analysed by means of a light optical microscope in order to evaluate the presence of cracks and their paths. The results can be summarized as follows: • The chemical composition of the bath strongly influences the zinc coating phases formation; • Bending tests confirm that the phases influence the resistance of specimens mainly at high values of deformation; • In the traditional coatings the crack starts at the iron-coating interface, propagates through the δ phase, and arrests in the ζ phase; • In coating characterized by Ti addition, the radial cracks are present in the δ phase. Sometime s delamination between δ and three phases zone is present; • τ phase is chara cterized by intergranular cracks.